1. Field of the Invention
The present invention relates to feedback control systems, and in particular to a control system for maintaining a constant concentration of a chemical being injected into a flow stream having fluctuating flow rates.
2. The Prior Art
One prior art method for automatically controlling the concentration of a chemical in a flow stream uses an ion-selective electrode, a data acquisition system, and specialized computer software. However, the electrodes used in this method often encounter interference from other ionic species, and thus this method is only capable of measurement within a specific limited range of ionic concentrations.
Although the method of the invention is obviously not limited to such an application, an important application is in the treatment of water channels with pesticides in order to destroy, and prevent growth of aquatic site pests such as Sago Pondweed. The current method of controlling the amount of such pesticides injected into a canal over time involves manually adjusting a peristaltic pump. However, many of the canals requiring aquatic site pest control are in areas where flood or spray irrigation is used. Each time water is added or removed from the canal, the flow rate of the canal changes. For this reason, the current method requires continuous monitoring and adjusting of the rate of pesticide injection. Sometimes the pesticide is most effective when injected over a period of 100 hours or more, which necessitates extensive manpower. Not only is this extensive manpower costly, but experience has shown that manual injection of the pesticide is often ineffective and costly as well because it is difficult to maintain proper pesticide concentration in this way.
Another candidate method for controlling the rate of chemical pesticide injection uses a personal computer along with a data acquisition system to monitor canal flow rate or pesticide concentration. A computer program uses this acquired data in order to control the peristaltic pump. However, this method is impractical for the specific applications being considered for at least the following reasons:
1) the majority of the sites where the injection system is to be used are remote and do not have access to the electrical grid power necessary to run the computer or data acquisition system and therefore, a large battery bank or a gas generator is required to meet the high energy demands; PA1 2) it is difficult and expensive to protect a computer and data acquisition equipment from potential vandalism (particularly where left unattended on site for extended periods) and the harmful effects of dust and moisture; PA1 3) highly trained, experienced personnel are required to set up and maintain both the computer and data acquisition system; and PA1 4) the associated capital costs are high.